SIMBAD references

Using the Rossi X-ray Timing Explorer and the Nordic Optical Telescope, we have obtained the highest ever quality X-ray/white-light high-speed photometry of XB1916-053. We refine the X-ray period (P_X) to 3000.6±0.2s via a restricted cycle counting approach. Using our complete optical light curve, we have extended the optical period (P_opt) ephemeris by another 4yr, providing further evidence for its stability, although a slightly longer period of 3027.555±0.002s now provides a marginally better fit. Moreover, modulations at both P_X and P_opt are present in the optical data, with the former dominating the nightly light curves (i.e. a few cycles of data). We have also attempted to determine the `beat' period, as seen in the repeating evolution of the X-ray dip structure, and the variation in primary dip phase. We find that a quasi-period of 4.74±0.05d provides the best fit to the data, even then requiring phase shifts between cycles, with the expected 3.90-d `beat' of P_X and P_opt appearing to be less likely. Finally, considering the nature of each of these temporal phenomena, we outline possible models, which could explain all of the observed behaviour of this enigmatic source, focusing on which of P_X or P_opt is the binary period.